The subject invention pertains to data processing systems, and more specifically to the communication channels, or interfaces, which couple together the various system components. As is known in the art data processing systems are composed of several units each having a specific function, such as a central processor, a working memory, and peripheral devices, connected to each other by means of a set of leads over which a complex system of electrical signals is transferred. A specific meaning and function is attributed to each signal, depending upon the particular lead used for its transmission. The set of these leads and signals, together with the set of electrical, logical, and procedural characteristics adhered to by these signals, is called a connection interface.
These interfaces comprise a considerable number of leads, posing serious problems of space and cost, due to both the dimensions and cost of the cables themselves, and to the dimensions and cost of the lead terminations; i.e. of the connectors and of the electrical control circuits.
For a long period attempts have been made to remedy these disadvantages by means of many different solutions.
For example, a circuit for the bidirectional transmission of interlocked signals has been disclosed in the Italian patent application 21510A/78, filed by the assignee of the instant application, and the corresponding U.S. patent application Ser. No. 023,050, filed Mar. 22, 1979. Such circuit uses a single lead to transfer a signal in one direction and a response signal in the opposite direction. A first embodiment of the above mentioned bidirectional transmission circuit enables the execution of a dialogue, which may be considered as being totally interlocked, and therefore especially reliable from a functional standpoint. However, such embodiment is relatively complex, inasmuch as it requires the transmission on the line of four distinct leading edges in order for the dialogue to be completed.
In a second embodiment of the above-mentioned circuit, the dialogue is simplified because it is executed by the transmission along the line of only two leading edges.
However, the latter dialogue is only partially interlocked, and at least one of the two interconnected units must provide an autonomous resetting means in order to reset the entire transmission circuit to its initial condition.
Such resetting means utilizes a delay line, and, therefore, has the disadvantage that the duration of the resetting operation is fixed. Thus, the duration of the resetting operation cannot be modified or optimized according to the length of the connection between two interconnected units. Furthermore, the device which is autonomously reset does not provide to the other device any information whatsoever for use by the latter to recognize the status, and possibly the resetting of the former.